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Recent discovery of correlated electronic phases in twisted heterostructures raised a surge of interests in studying models and materials with flat bands where the electronic excitations are nearly dispersionless in momentum space. As such, the kinetic energy is quenched and the correlations are enhanced, giving rise to a plethora of unusual magnetic, superconducting and transport behaviors. Finding materials whose energy bands are completely flat is rather challenging, yet those whose dispersion is flat only in a portion of the momentum space might be more accessible in material search. In this work, we propose a partially flat-band system on a square lattice. Using the Hubbard model, it is demonstrated that the suppression of the electronic kinetic energy in the flat portion of the band dispersion drives the system to Stoner ferromagnetism even at very weak interactions, i.e., much smaller than the bandwidth, with significantly enhanced Curie temperature. While the low-energy magnon modes are well defined collective excitations, flat magnon bands can be observed at high energies. We show that the strong interaction leads to reduction of the flat portion of the magnon band. However, tuning the chemical potential at a strong interaction regime may lead to spin density wave at finite wave vectors. Then, focusing on the non-magnetic correlated phase and using dynamical mean-field theory, we demonstrate the appearance of a flat-band induced sharp peak in the density of states in addition to the correlation-induced Mott bands. Furthermore, the large seebeck coefficient and the figure of merit of the proposed partially flat-band model, compared to symmetric regular band models, put them in the category of efficient thermoelectric materials., Comment: 11 pages, 11 figures

The broad tunability of the proximity exchange effect between transition-metal dichalcogenides (TMDCs) and chromium iodide (CrI$_3$) heterostructures offers intriguing possibilities for the use of TMDCs in two-dimensional magnetoelectrics. In this work, the influence of the twist angle and the gate electric field on the electric and thermal transport in a TMDC/CrI$_3$ junction is investigated using the Dirac -Bogoliubov-de Gennes equation. We show that significant amounts can be controlled by spin-splitting of band structures due to spin-orbit interaction, and that the exchange-splitting of bands arises from the proximity effect. The property of the Andreev reflection (AR) process is highly dependent on the spin valley polarized states due to spin-orbit coupling. Remarkably, perfect spin valley polarized AR is possible over a wide bias range by using a gate voltage to tune the local Fermi energy and varying the type of charge doping. The proposed structure with $p$-type doping is found to have larger spin valley polarized Andreev conductance and high thermal conductance. We further show that, depending on the TMDC material and chemical potential of the TMDC/CrI$_3$ layer, twisting can lead to suppression or a significant increase in Andreev conductance as well as enhancement of thermal conductance for chemical potentials smaller than that of the superconducting regime., Comment: 14 pages, 10 figures

We theoretically reveal the unusual features of the Josephson effect in a strained zigzag graphene nanoribbon with a small length relative to the superconducting coherence length and an arbitrary width. We find a step-wise variation of the critical supercurrent with the width of the nanoribbon, showing additional small width plateaus placed between the broad steps of a unstrained structure. We further demonstrate the peculiar quantization of the critical supercurrent in terms of the strain, resulted from the coupling of the pseudospin of Dirac fermions with the strain-induced gauge potential, where the height of the steps decreases with growing the strength of the fictitious gauge potential. Moreover, our results determine the potential of the proposed superconducting quantum point contact for the realization of the supercurrent switch under an applied strain. Besides, we find the local density of states of the strained zigzag nanoribbon displays a crossover between the decaying and oscillating behavior with the distance from the edges, by tuning the width and Fermi wavelength of the nanoribbon., Comment: 23 pages, 9 figures. Accepted for publication in New Journal of Physics

We theoretically investigate the unusual features of the magnetotransport in a monolayer phosphorene ferromagnetic/normal/ferromagnetic (F/N/F) hybrid structure. We find that the charge conductance can feature a minimum at parallel (P) configuration and a maximum near the antiparallel (AP) configuration of magnetization in the F/N/F structure with $n$-doped F and $p$-doped N regions and also a finite conductance in the AP configuration with the N region of $n$-type doping. In particular, the proposed structure exhibits giant magnetoresistance, which can be tuned to unity. This perfect switching is found to show strong robustness with respect to increasing the contact length and tuning the chemical potential of the N region with a gate voltage. We also explore the oscillatory behavior of the charge conductance or magnetoresistance in terms of the size of the N region. We further demonstrate the penetration of the spin-transfer torque into the right F region and show that, unlike graphene structure, the spin-transfer torque is very sensitive to the chemical potential of the N region as well as the exchange field of the F region., Comment: 10 pages, 7 figures

We theoretically reveal the possibility of specular Andreev reflection in a thin film topological insulator normal-superconductor (N/S) junction in the presence of a gate electric field. The probability of specular Andreev reflection increases with the electric field, and electron-hole conversion with unit efficiency happens in a wide experimentally accessible range of the electric field. We show that perfect specular Andreev reflection can occur for all angles of incidence with a particular excitation energy value. In addition, we find that the thermal conductance of the structure displays exponential dependence on the temperature. Our results reveal the potential of the proposed topological insulator thin-film-based N/S structure for the realization of intraband specular Andreev reflection., Comment: 10 pages, 9 figures. Some typos are corrected

We propose a valley- and spin-filter based on a normal/ferromagnetic/normal molybdenum disulfide (MoS$_2$) junction where the polarizations of the valley and the spin can be inverted by reversing the direction of the exchange field in the ferromagnetic region. By using a modified Dirac Hamiltonian and the scattering formalism, we find that the polarizations can be tuned by applying a gate voltage and changing the exchange field in the structure. We further demonstrate that the presence of a topological term ($\beta$) in the Hamiltonian results in an enhancement or a reduction of the charge conductance depending on the value of the exchange field., Comment: 11 pages, 5 figures. arXiv admin note: text overlap with arXiv:1402.1840

We propose a hole-doped molybdenum disulfide (MoS$_2$) superconducting spin valve (F/S/F) hybrid structure in which the Andreev reflection process is suppressed for all incoming waves with a determined range of the chemical potential in ferromagnetic (F) region and the cross-conductance in the right F region depends crucially on the configuration of magnetizations in the two F regions. Using the scattering formalism, we find that the transport is mediated purely by elastic electron cotunneling (CT) process in a parallel configuration and changes to the pure crossed Andreev reflection (CAR) process in the low-energy regime, without fixing of a unique parameter, by reversing the direction of magnetization in the right F region. This suggests both valley- and spin-switch effects between the perfect elastic CT and perfect CAR processes and makes the nonlocal charge current to be fully valley- and spin-polarized inside the right F region where the type of the polarizations can be changed by reversing the magnetization direction in the right F region. We further demonstrate that the presence of the strong spin-orbit interaction $\lambda$ and an additional topological term ($\beta$) in the Hamiltonian of MoS$_2$ result in an enhancement of the charge conductance of the CT and CAR processes and make them to be present for long lengths of the superconducting region. Besides, we find that the thermal conductance of the structure with a small length of the highly doped superconducting region exhibits linear dependence on the temperature at low temperatures whereas it enhances exponentially at higher temperatures. In particular, we demonstrate that the thermal conductance versus the strength of the exchange field ($h$) in F region displays a maximum value at $h<\lambda$, which moves towards larger exchange fields by increasing the temperature., Comment: 13 pages, 8 figures

Andreev reflection in a monolayer molybdenum disulfide superconducting-normal (S/N) hybrid junction is investigated. We find, by using a modified-Dirac Hamiltonian and the scattering formalism, that the perfect Andreev reflection happens at normal incidence with $p$-doped S and N regions. The probability of the Andreev reflection and the resulting Andreev conductance, in this system, are demonstrated to be large in comparison with corresponding gapped graphene structure. We further investigate the effect of a topological term ($\beta)$ in the Hamiltonian and show that it results in an enhancement of the Andreev conductance with $p$-doped S and N regions, while in the corresponding structure with $n$-doped S region it is strongly reducible in comparison. This effect can be explained in terms of the dependence of the Andreev reflection probability on the sign of $\beta$ and the chemical potential in the superconducting region., Comment: 8 pages, 6 figures, accepted for publication in Phys. Rev. B

We investigate the unusual features of the quantum transport in gapped monolayer graphene, which is in a pseudospin symmetry-broken state with a net perpendicular pseudomagnetization. Using these pseudoferromagnets (PFs), we propose a perfect pseudospin valve effect that can be used for realizing pseudospintronics in monolayer graphene. The peculiarity of the associated effects of pseudo spin injection and pseudo spin accumulation are also studied. We further demonstrate the determining effect of the sublattice pseudospin degree of freedom on Andreev reflection and the associated proximity effect in hybrid structures of PFs and a superconductor in S/PF and PF/S/PF geometries. In particular, we find a peculiar Andreev reflection that is associated with an inversion of the z component of the carriers pseudospin vector. Our results show that the gapped normal graphene behaves like a ferromagnetic graphene and the effect of the pseudospin degree of freedom in gapped graphene is as important as the spin in a ferromagnetic graphene., Comment: 19 pages, 10 figures. Accepted for publication in the Journal of Computational Electronics special issue on "Electronic transport in graphene nanostructures and devices". A review of two submissions (arXiv:1208.2461, arXiv:1110.0637) + additional results for the pseudospin injection in pseudoferromaget/normal graphene junctions and the superconducting pseudospin valves

We theoretically demonstrate unusual features of superconducting proximity effect in gapped graphene which presents a pseudospin symmetry-broken ferromagnet with a net pseudomagnetization. We find that the presence of a band gap makes the Andreev conductance of graphene superconductor/pseudoferromagnet (S/PF) junction to behave similar to that of a graphene ferromagnet-superconductor junction. The energy gap $\Delta_N$ enhance the pseudospin inverted Andreev conductance of S/PF junction to reach a limiting maximum value for $\Delta_N\gg \mu$, which depending on the bias voltage can be larger than the value for the corresponding junction with no energy gap. We further demonstrate a damped-oscillatory behavior for the local density of states of the PF region of S/PF junction and a long-range crossed Andreev reflection process in PF/S/PF structure with antiparallel alignment of pseudomagnetizations of PFs, which confirm that, in this respect, the gapped normal graphene behaves like a ferromagnetic graphene., Comment: 7.2 pages, 5 figures, accepted for publication in Phys. Rev. B

Monolayer graphene with an energy gap presents a pseudospin symmetry broken ferromagnet with a perpendicular pseudomagnetization whose direction is switched by altering the type of doping between n and p. We demonstrate an electrical current switching effect in pseudospin version of a spin valve in which two pseudoferromagnetic regions are contacted through a normal graphene region. The proposed structure exhibits a pseudomagnetoresistance, defined as the relative difference of resistances of parallel and antiparallel alignments of the pseudomagnetizations, which can be tuned to unity. This perfect pseudomagnetic switching is found to show a strong robustness with respect to increasing of the contact length, the effect which we explain in terms of an unusually long range penetration of an equilibrium pseudospin polarization into the normal region by proximity to a pseudoferromagnet. Our results reveals the potential of gapped graphene for realization of pseudospin-based nanoelectronics., Comment: 6 pages, 5 figures